1. Field of the Invention
The invention relates to data transmission via frequency-shift keying, and more particularly to a method and system for fast and reliable encoding and decoding of arbitrary frequency-shift keying signals.
2. Brief Description of the Prior Art
It is common to provide electronic data terminal equipment, such as computers, printers, etc., located at some distance from each other, and which interact with each other to exchange information. For the exchange of data, it is necessary to use a physical representation of the data in the form of signals adapted to the particular transmission link. It is known to transmit data by means of a time sequence of electrical currents or voltages. If cables are used as the transmission link, both ac and dc signals can be used. Only ac signals are, however, possible for communications channels such as telephone and radio. For transmission of data, a unique pattern representing information is impressed on a carrier at the transmitter end, which must be recognized at the receiver end. Problems are present in pattern recognition at the receiver end since non-ideal communications channels usually distort the impressed pattern due to channel characteristics. For example, a typical speech channel ordinarily has a bandwidth of only about 300 Hz to 3400 Hz, a frequency-dependent amplitude response and phase response, and frequency-shifts of a few Hertz.
The ac patterns are generated primarily by means of a controlled manipulation of one or more of the three parameters: phase, amplitude, and frequency of sinusoidal oscillations. In the QAM method (quadrature amplitude modulation) which is used in accordance with CCITT V.29 and V.32, the phase and amplitude of a constant-frequency sinewave are varied. In frequency-shift keying (FSK) methods, which are applied in accordance with CCITT V.21 for example, a sinewave is switched stepwise from one frequency to another frequency at zero crossings or at peaks, the transition thus being effected without a phase jump.
The fundamental encoding formula for frequency-shift keying is simple: one digital bit combination is represented by one frequency and another digital bit combination is represented by another frequency. A particular frequency in the communications channel at a particular time thus corresponds exactly to one bit combination.
The reliability of frequency-shift keying methods is relatively high because phase and amplitude variations that may occur in non-ideal communications channels have no effect on the information content of the signal. The disadvantage of FSK methods so far is a rather low transmission speed (e.g., 300 bits/second according to CCITT V.21). This is explained primarily by the restricted frequency band of about 3100 Hz of speech channels. A faster switching between the frequencies for the bit combinations according to decoding techniques known heretofore requires, however, an even larger frequency interval in order to be able to recover the information reliably at the receiver end.
Prior art decoding techniques in FSK methods commonly use analog or digital phase-locked loops (PLLs) which have relatively long transient response times. More recent versions of frequency-shift keying methods (e.g., minimum-shift keying (MSK) and fast frequency-shift keying (FFSK)) exploit the fact that frequencies are represented by zero crossing intervals, and use zero crossing detectors in combination with counters as frequency discriminators. However, these count values do not reproduce the frequencies correctly, especially near the keying times. Namely, in the time range of the neighboring frequencies the originally time-limited frequencies ordinarily exhibit signal residues (caused by varying group delays, among other things) which lead to mutual interference. Therefore, this fact has made it necessary to choose the intervals of the keying times relatively large in comparison with the potentially usable keying times, so that even in the more recent of frequency-shift keying methods, a comparatively low transmission speed of at most 2400 bits/second has been possible.
The problem addressed by the invention is to improve the encoding and decoding technique for frequency-shift keying methods of the above-cited type so that, in particular, frequencies being defined near the keying times are recognized quickly and reliably, and to present a system for implementation of the method that can be produced economically, has low susceptibility to interference, and which makes possible fast and reliable transmission of data in non-ideal communications channels.